20 COMPARATIVE PHYSIOLOGY. 



are much more resistant than the mature organisms themselves. 

 Some spores can resist a dry heat of 140° C. 



The spores, like Torula and Protococcus, bear drying, with- 

 out loss of vitality, for considerable periods. 



That different groups of bacteria have a somewhat diflferent 

 life-history is evident from the fact that the presence of one 

 checks the other in the same fluid, and that successive swarms 

 of different kinds may flourish where others have ceased to 

 live. 



That these organisms are enemies of the constituent cells of 

 the tissues of the highest mammals has now been abundantly 

 demonstrated. That they interfere with the normal working 

 of the organism in a great variety of wajrs is also clear ; and 

 certain it is that the harm they do leads to aberration in cell- 

 life, however that may be manifested. They rob the tissues of 

 their nutriment and oxygen, and poison them by the products 

 of the decompositions they produce. But apart from this, their 

 very presence as foreign agents must hamper and derange the 

 delicate mechanism of cell-life. 



These organisms seem to people the air, land, and waters 

 with invisible hosts far more numerous than the forms of life 

 we behold. Fortunately, they are not all dangerous to the 

 higher forms of mammalian life ; but that a large proportion 

 of the diseases which afBict both man and the domestic animals 

 are directly caused by the presence of such forms of life, in the 

 sense of being invariably associated with them, is now beyond 

 doubt. 



The facts stated above explain why that should be so ; why 

 certain maladies should be infectious ; how the germs of dis 

 ease may be transported to a friend wrapped up in the folds of 

 a letter. 



Disease thus caused, it must not be forgotten, is an illustrar 

 tion of the struggle for existence and the survival of the fittest. 

 If the cells of an organism are mightier than the bacteria, the 

 latter are overwhelmed ; but if the bacteria are too great in 

 numbers or more vigorous, the cells must yield ; the battle may 

 waver — now dangerous disease, now improvement — but in the 

 end the strongest in this, as in other instances, prevail. 



